The structure and biosynthesis of the mitochondrial ATPase complex, a key enzyme in oxidative phosphorylation has been investigated in yeast. The complex has been resolved into three functional units: 1) a catalytic ATPase unit F1; 2) a membrane factor modulating the enzymatic and physical properties of F1 and; 3) OSCP a protein that links F1 to the membrane factor. The four proteins of the membrane factor have been shown to be synthesized on mitochondrial ribosomes whereas OSCP and the five subunits of F1 are synthesized on cytoplasmic ribosomes. Mutants of S. cerevisiae have been isolated which are specifically affected in the ATPase. Two such nuclear mutants have been found to have defects in one of the major subunits of F1. Neither of these mutants exhibit any ATPase activity. The role of the defective subunit in the catalytic activity of F1 is also supported by immunochemical evidence employing antisera to the purified subunits of the enzyme. Another mutant in which the genetic lesion has been found to be associated with mitochondrial DNA has also been studied. This mutation is closely linked to the OII locus of mitochondria DNA which is expressed by a resistance of the enzyme to oligomycin. The identity of the aberrant protein has not yet been established. Bibliographic references: R.B. Needleman and A. Tzagoloff (1975), A Rapid Method for Breakage of Yeast, Anal. Biochem. 64, 545; A. Tzagoloff, A. Akai, and R.B. Needleman (1975), Assembly of the Mitochondrial Membrane System XII. Cytoplasmic Mutants of Yeast with Defects in Cytochrome Oxidase, Proc. Natl. Acad. Sci. U.S. 72, 2054.